In prior pipeline stopper assemblies of the character disclosed in the aforementioned U.S. Pat. No. 3,543,802, the assembly utilized sets of angularly spaced toggle arms connected between a cylindrical longitudinally split expanding sleeve and members movable coaxially of the sleeve. The members were threadedly engaged with a jackscrew and rotation of the jackscrew effected movement of the members toward one another to expand the sleeve or away from one another to contract the sleeve. Stop means were provided to limit the movement of the toggle arms so that their pivotal movement was merely to an angle no greater than 90.degree. to the longitudinal axis of the stopper assembly thereby ensuring positioning and proper operation for a particular size stopper assembly when used with a particular size pipeline. However, these stop means engaged the toggle arms directly and continued actuation of the jackscrew could place undesirable stresses or loads on the toggle arms and their pivots possibly damaging the same and resulting in an inoperative stopper assembly or possibly damaging the seating surface of the cylindrical split sleeve.
In previous stopper assemblies, the split sleeve was held from rotating by the use of a second shaft on the stopping machine and while this resulted more mechanism being incorporated into the stopping machine, it also did not ensure proper positioning of the stopper assembly in the transverse opening through a pipeline.
With the advent of the use of large diameter pipes of 20 inches, 30 inches and more, the prior art pipeline stopper assemblies had to be constructed considerably larger in size to accommodate the increased size of the transverse openings required for such large diameter pipelines. By utilizing a pipeline stopper design of the type disclosed in the aforementioned U.S. Pat. No. 3,543,802, there was a great weight sacrifice necessary to make the toggle arms stiff and strong enough to resist the torque transferred from the jackscrew through the operating nuts to the toggle arms and to the longitudinally split sleeve. While the toggle arms had to not only be increased in size, their pivots to the operating nut members and to the sleeve member also had to be increased in size and weight to provide the necessary strength required because of increasing loads and torques. The only other solution to the problem created by designing larger pipeline stopper assemblies was to use expensive and exotic high strength metals capable of transferring the torques and loads through the toggle arms from the operating nut members to the longitudinally split sleeve but this solution was impracticable because of the expenses involved in the manufacture of such units.